Typically, modern wind turbines have rotor blades with adjustable pitch angle. The rotor blades can be rotated about their longitudinal axis by means of a pitch drive disposed in the rotor hub. Typically, the pitch drive is actuated electrically or hydraulically. By adjusting the pitch angles of the rotor blades, the power generation of the wind turbine can be controlled as well as an aerodynamical braking of the rotor can be accomplished. Particularly, the rotor blades generate a braking torque when moved into feather position. Thereby, the rotor blades ensure that the rotor is not further accelerated and, thus, the rotor blades form an aerodynamical brake for the wind turbine.
The aerodynamical brake effect of the rotor blades is also used for braking the rotor in case of emergency, e.g. when a failure of the drive system occurs or when the wind turbine is in an uncontrolled condition. Accordingly, it is very important that the emergency system works absolutely reliable even when the wind turbine is damaged or in an uncontrolled state, e.g. due to lightning. Especially, the pitch drive is required to function even in a power outage condition.
As a solution to this problem, uninterruptible power supplies (UPS) for the pitch drive have been proposed, e.g. in chapter 8.4.6 of “Windturbines” by E. Hau, Springer-Verlag, 2000, pages 236 and 237. However, such an approach may fail when the electronics of the wind turbine is damaged, e.g., by lightning.
Therefore, a redundant mechanical fallback system has been proposed in DE 197 20 025. Therein, the pitch drive is actuated by energy stored in springs which are coupled to the drive shaft of the pitch drive via a gear. According to DE 197 20 025, several springs are accommodated in a casing with separator disks in between. The springs are fixed to the casing to a center shaft extending through the casing with hooks, respectively. However, the arrangement according to DE 197 20 025 does not ensure smooth operation of the springs: the separator disks moved vertically along the center shaft and, thus, laterally pinched the springs. It was also observed that the hook fixed to the casing broke during operation. The hook was fixedly attached to the casing and, therefore, immense mechanical stress was applied to the hook during winding up the spring. Finally, the operational safety of the arrangement according to DE 197 20 025 is insufficient: In the event that a tightening strap of a spring broke, the spring exploded and could hurt somebody. Also, the spring was unusable afterwards. Furthermore, the monitoring of the spring condition is accomplished by measuring the diameter of the spring coil. However, this requires complicated sensor equipment which is error-prone.